low power AM transmitter

Discussion in 'The Projects Forum' started by maliha tamkeen, Jan 6, 2016.

  1. maliha tamkeen

    Thread Starter New Member

    Jan 6, 2016
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    We are trying to do this experiment for the school Science fair.
    http://www.sciencebuddies.org/science-f ... #procedure

    We got all the products (except batteries and audio cable) from Science buddies .We used regular headphones as the phone cable.We used the radio (530kHz-1700kHz) as the reciever.There is no output.
    Should we change the phone cable?
    As per the procedure

    (Use your wire strippers to cut the 3.5 mm audio cable in half. It has three wires inside: left and right audio (with red and white insulation), and ground (uninsulated).
    Strip about 5 mm of insulation off the ends of the left and right audio wires.)

    What else could be the problem?
    We tried putting only one end of earphone in A5.We get quieting of the background noise at 1MHz on the home radio,but no other output.I am hoping that there is RF at the output,but what should we do to hear the output?
    I tried reversing the transformer ,but did not help.I believe we have used 1000 ohms to 8 ohms transformer. Are we not supposed to use step up transformer(8 ohms to 1000 ohms).
    Also we used 4 AA rechargeable batteries of 1.2v each.The specification of Crystal Oscillator mention 5V input voltage.Can that be the problem?
    We live in Fremont,CA-94538.Can the local AM radio signal be the problem.There is a radio station at 1010 kHz frequency from San Francisco. But I can clearly hear blank when I turn on and noise when I turn off the battery,so I think we are catching the signal.Can you please search for any local signals at 1MHz in my area.
    We tried computer ,IPad and Ipod as the sound source.What can we do intensify the input signal?Do you think we should try with the 3.5mm audio cable?
    Should I send picture of my circuit .May be that will help.
    Please let know.
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
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    Yes, please post your circuit.
     
  3. maliha tamkeen

    Thread Starter New Member

    Jan 6, 2016
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  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Please post a photo of your construction. You might have the transformer connected incorrectly. I was going to suggest pulsing the power to see if you could hear the carrier come and go on a radio, but you already did that (1 gold star).

    This circuit "misuses" the crystal oscillator, so it might not be happy having its power supply bouncing around with audio on it.

    ak
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    Bad wring, bad parts, terrible luck, bad radio, ....

    Showing what you have and comparing it vs. the schematic would be the first step.
     
  6. maliha tamkeen

    Thread Starter New Member

    Jan 6, 2016
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    RF006.jpg RF007.jpg
    Mod Edit: Reduced file sizes, removed duplicate images
     
    Last edited by a moderator: Jan 6, 2016
  7. BR-549

    Well-Known Member

    Sep 22, 2013
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    It's hard to tell with my eyes, do you have complete audio circuit from phone connected to transformer?
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    1. Make sure that the red and blue wires on the right side of the xfmr are in the same rows as the transformer pins.

    2. Make sure that the center conductor and shield wires on the left side of the xfmr are in the same rows as the transformer pins.

    3. Make sure the transformer pins are engaged deeply enough into the proto board to make contact with their rows. Close up photos looking straight into each side of the transformer would help.

    ak
     
  9. JohnInTX

    Moderator

    Jun 26, 2012
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    @wayneh Posted something very similar in Completed Projects here.
    I built one and was able to modulate the RF just fine. It transmitted over the air just fine (and probably illegally) too.
    Good luck.
    BTW, if you post pictures, please try to keep the size around 300K-400K. We have many users on slower connections.
    Thanks.
     
  10. DickCappels

    Moderator

    Aug 21, 2008
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    The problem appears to be that of modulation. Can't tell from the photograph, but is the audio from the cell phone in series with the transformer and battery? You can check by unplugging the cable from the phone and checking to see whether there is 6 volts between the sleeve (longest contact on the plug) and the ring or tip (the other two contacts).
     
  11. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    First off -- Please follow the SXO with a low-pass filter! - As shown, odd harmonics will be detectable (and likely cause QRM/RFI) well into the VHF region!
    Secondly -- Make certain the modulation source is adjusted such that Vcc/Vdd extremes remain within specifications!
    Thirdly -- Please expect some FM/PM -- The fact that the unit is internally buffered does not mitigate the consequences of 'AMing' the oscillator stage...

    Good luck!
    HP:)
     
    Last edited: Jan 7, 2016
  12. sailorjoe

    Member

    Jun 4, 2013
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    From the photos it appears you didn't strip your headphone wires properly, or perhaps they are not stereo headphones.

    Return to the instructions, and review the description of how to strip the wires. Each individual wire is actually two wires, one running down the center, and the other as a braided wire sheath around the center. This is all then wrapped in plastic. The sheath is called the "shield". You need to remove an inch of the outer plastic to expose the shield. Then carefully unwrap the shield into a circle of straight strands around the wire. Then twist all the strands back together into an approximation of fat wire. Its delicate work.

    Meanwhile, keep your center conductor, the one shown, free from the shield. When you're ready, plug in both the center conductor and the shield conductor into your circuit board as instructed.
     
  13. Aleph(0)

    Member

    Mar 14, 2015
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    Ha ha! Wise woman of the forum say: He who has time for experimental toy must make laPLACE for Fourier:D:p
     
  14. dannyf

    Well-Known Member

    Sep 13, 2015
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    What type of a transformer did you use?

    You could also have wired the transformer in backwards. In that case, just reverse the transformer.

    All the transformer did is to match impedance. You could have done that will a transistor.
     
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  15. Aleph(0)

    Member

    Mar 14, 2015
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    That's true but that transmitter has so much room for improvement there is no telling where to starto_O! Maliha Tamkeen I say you should use a pierce osc with isolation and power amp stage and modulate just the PA! Since you're intent on causing interference at least you'd be holding it to a _single_ frequency:rolleyes:!
     
    Hypatia's Protege likes this.
  16. Hypatia's Protege

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    Mar 1, 2015
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    Indeed! I too am appalled that a 'science project' publication should encourage 'square wave carriers':confused::eek::mad: -- That said, I have to wonder if a clean QRO pirate is preferable a noisy QRP hobbyist???

    Best regards
    HP:)
     
  17. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    No, it doesn't. It modulates the Vcc supply to the oscillator with the audio waveform. Basically, the crystal oscillator is powered by two power sources in series, one constant and one varying. Because the oscillator output stage saturates to Vcc and GND, this causes the audio to modulate the amplitude of the output square wave. This is the original form of amplitude modulation, used before the vacuum tube was invented and for years after until an electronic modulator had enough power to be worth anything. Replace the crystal oscillator with a GE motor-driven multi-multi-pole AC generator making 50 kHz, and you have what Fessenden used for the world's first entertainment audio radio broadcast.

    Despite all of the grumbling about the square wave carrier (it's what, 5 mW? Calm down.), I think this is a great little project. It covers all of the basics of AM without intimidating an amateur with base biasing (and a Pierce oscillator - ???). This is the kind of thing that creates a spark.

    ak
     
  18. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    I think this is what @Aleph(0) had in mind (if not, I know I'll hear about it!:rolleyes:) -- Note, however, that in this example the Caps may be a bit 'scant' for 1MHz operation...
    [​IMG]

    Modulating the oscillator!? Don't you mean AM, FM plus 'intermod' and 'crossmod';);):D

    Seriously I get your point:) - and I can well imagine the difficulties attendant to catching and holding the "student's" interest --- Even so, construction of a proper 'flea-power' AM transmitter isn't all that demanding and the experimenter comes away with a skill -- as opposed to a 'trick' --- Just my $.02

    Best regards
    HP:)
     
  19. dannyf

    Well-Known Member

    Sep 13, 2015
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    Everything has much room for improvement, more so with the commercial transmitters used in real radio stations.

    But that's not the point. This simple contraption works: I have used in AM band and in SW bands too. The issue here is user error (errors), not the conceptual soundness of this design.

    As to "square wave", you should put your scope on the output and you may be surprised at how "unsquare" wave they are.
     
  20. BReeves

    Member

    Nov 24, 2012
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    As already mentioned, looks to me like the blue jumper isn't connected to the transformer.
     
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